1. Field of the Invention
The present invention relates to a solid electrolytic capacitor in which a dielectric coating and a solid electrolyte layer are sequentially formed on a surface of an anode element, and to a manufacturing method therefor.
2. Description of Related Art
Solid electrolytic capacitors are small and large-capacity in addition to being excellent in high-frequency characteristics, and therefore widely used in high-frequency circuits of various electronic devices such as personal computers and imaging devices.
FIG. 14 shows one example of a most typical solid electrolytic capacitor. This solid electrolytic capacitor 1 includes a capacitor element 2 having a function as a capacitor. The capacitor element 2 is formed with a block-like anode element 3 serving as a base. The anode element 3 is a sintered body of a valve-action metal such as tantalum, niobium, titanium or aluminum. A rod-like anode lead member 4 projects from an end face of the anode element 3. The anode lead member 4 is made of a valve-action metal of tantalum.
A dielectric coating 5 is formed on a surface of the anode element 3 and a surface of the anode lead member 4 near the anode element 3. The dielectric coating 5 is formed by oxidizing the surfaces of the anode element 3 and the anode lead member 4 with an anodic oxidation method, for example. A solid electrolyte layer 7 is formed on the dielectric coating 5. The solid electrolyte layer 7 is made of a conductive inorganic material such as manganese dioxide, or a conductive organic material such as TCNQ complex salt and a conductive polymer. A cathode lead layer is formed on the solid electrolyte layer 7. The cathode lead layer includes a carbon layer 8 and a silver layer 9, for example. A plate-like anode terminal 10 is connected to the anode lead member 4, while a plate-like cathode terminal 11 is connected to the cathode lead layer. The capacitor element 2 is coated with an enclosure member 12. The enclosure member 12 is generally in the form of a rectangular parallelepiped. The enclosure member 12 is made of epoxy resin, for example. The anode terminal 10 and the cathode terminal 11 are drawn out from the enclosure member 12 in opposite directions, and bent downward. The leading ends of these terminals 10, 11 are arranged along the lower surface of the enclosure member 12, and used for soldering the solid electrolytic capacitor to a mounting board.
In a manufacturing method for the solid electrolytic capacitor 1 as described above, the solid electrolyte layer 7 is formed by a chemical polymerization or an electrolytic polymerization method when a conductive polymer such as polypyrrole is used for the solid electrolyte layer 7.
In the chemical polymerization method, the solid electrolyte layer 7 is formed by oxidation-polymerizing a monomer using an oxidizing agent. Stated more specifically, the dielectric coating 5 is formed on the surfaces of the anode element 3 and the anode lead member 4, and thereafter the oxidizing agent is attached on the dielectric coating 5. Then, the anode element 3 and the anode lead member 4 having the oxidizing agent attached thereto are dipped in a solution having the monomer dissolved therein, or left in the monomer atmosphere. In this way, the monomer is polymerized on the dielectric coating 5 to form the solid electrolyte layer 7.
On the other hand, in the electrolytic polymerization method, the dielectric coating 5 is formed on the surfaces of the anode element 3 and the anode lead member 4, and then a precoat layer 7a made of a solid electrolyte is formed on the dielectric coating 5 using the chemical polymerization method as described above. Then, the anode element 3 and the anode lead member 4 having the precoat layer 7a formed thereon are dipped in a solution having a monomer dissolved therein. A tank with the solution therein is provided with an electrode and an electrode plate. With the electrode contacting the precoat layer 7a, a voltage is applied with the electrode serving as a positive electrode and the electrode plate as a negative electrode. In this way, the monomer is oxidation-polymerized to form a conductive polymer layer 7b covering the precoat layer 7a. 
There has been proposed a manufacturing method as described below for a solid electrolytic capacitor using a conductive polymer such as polypyrrole, polythiophene, polyaniline, etc. as a solid electrolyte layer 7 (JP 2005-045235 A). In this manufacturing method, as shown in FIG. 16, a dielectric coating 5 is formed on a surface of an anode element 3 provided with an anode lead member 4 projecting therefrom, and on a surface of the anode lead member 4 at the side of the anode element 3. Then, a precoat layer 7a is formed on the dielectric coating 5 using a chemical polymerization method. Thereafter, a process is performed for partially removing the dielectric coating 5 and the precoat layer 7a on the anode lead member 4 to expose the surface of the anode lead member 4. In the process, a laser beam, for example, is radiated to a part from which the dielectric coating 5 and the precoat layer 7a are to be removed.
Next, a conductive polymer layer 7b is formed on the precoat layer 7a. First, the anode element 3 and the anode lead member 4 are dipped in a solution having a monomer to be polymerized to be a conductive polymer dissolved therein. At this time, the exposed surface of the anode lead member 4 is positioned on the surface of the solution. Then, a voltage is applied between an electrode plate in the solution and the anode lead member 4. In this way, the conductive polymer layer 7b is formed on the exposed surface of the anode lead member 4 and on the precoat layer 7a. The conductive polymer layer 7b acts as a solid electrolyte layer 7 together with the precoat layer 7a. 
Thereafter, as shown in FIG. 17, a laser beam 31 is radiated to the conductive polymer layer 7b formed on the surface of the anode lead member 4 to remove the conductive polymer layer 7b formed on the surface of the anode lead member 4 and insulate the anode lead member 4 from the solid electrolyte layer 7. Thereafter, an injection molding process for forming an enclosure member 12 and an aging process are further performed. With such a manufacturing method, a manufacturing process can be more efficient because removal of burrs of the conductive polymer layer 7b generated by the electrolytic polymerization process and removal of an unnecessary part of the dielectric coating 5 and the solid electrolyte layer 7 on the anode lead member 4 can be performed simultaneously.
FIG. 15 shows a solid electrolytic capacitor manufactured by the above-described manufacturing method. In such a capacitor, end faces of a dielectric coating 5 and a solid electrolyte layer 7 formed on an anode lead member 4 are formed approximately flush with each other. On the other hand, it is difficult to form the end faces of the dielectric coating 5 and the solid electrolyte layer 7 on the anode lead member 4 approximately flush with each other in a conventional method in which removal of burrs of a conductive polymer layer 7b is mechanically performed using a file or grinder. Accordingly, as shown in FIG. 14, the end face of the dielectric coating 5 is located closer to the leading end of the anode lead member 4 than the end face of the solid electrolyte layer 7. The solid electrolytic capacitor shown in FIG. 15 can be made smaller than the conventional solid electrolytic capacitor shown in FIG. 14 because an anode terminal 10 can be connected to the anode lead member 4 near the anode element 3.
However, removal of burrs of the conductive polymer layer 7b using the laser beam 31 as shown in FIG. 17 results in a low strength near the end faces of the dielectric coating 5 and the solid electrolyte layer 7 because of heat of the laser beam 31. As shown in FIG. 15, a capacitor element 2 has a periphery thereof covered with an enclosure member 12 made of epoxy resin or the like. When the capacitor element 2 is heated during the injection molding process or aging process, a slight gap can occur between the end faces of the dielectric coating 5 and the solid electrolyte layer 7 and the enclosure member 12 because of a difference between a coefficient of thermal expansion of the enclosure member 12 and a coefficient of thermal expansion of the dielectric coating 5 and the solid electrolyte layer 7.
If a gap exists between the end faces of the dielectric coating 5 and the solid electrolyte layer 7 and the enclosure member 12, the dielectric coating 5 and the solid electrolyte layer 7 with a low strength generate cracks due to external stress or time degradation, causing a problem of an increase of a leak current or short circuit occurrence.
When the capacitor element 2 is heated during the injection molding process or aging process, a slight gap can occur also between the anode lead member 4 and the enclosure member 12 because of a difference between a coefficient of thermal expansion of the enclosure member 12 and a coefficient of thermal expansion of the anode lead member 4. If there is a gap between the enclosure member 12 and the anode lead member 4, then, under high humidity environment, external moisture is likely to enter an inside of the enclosure member 12 through the gap from an interface between the anode terminal 10 and the enclosure member 12. The moisture acquired on the end faces of the dielectric coating 5 and the solid electrolyte layer 7 causes a problem of an increase of a leak current or occurrence of a short circuit. Even if the end faces of the dielectric coating 5 and the solid electrolyte layer 7 on the anode lead member 4 are formed approximately flush with each other as shown in FIG. 15 without the laser beam, there is a problem of a leak current being easily increased because a distance between the anode lead member 4 and the solid electrolyte layer 7 becomes much shorter than that of the conventional solid electrolytic capacitor 1 shown in FIG. 14.